Electron beam focusing device



Jan. 26, 1960 P. MEYERER ELECTRON BEAM FOCUSING DEVICE Filed Aug. 20.1956 lni l' United States Patent ELECTRON BEAM FOCUSING DEVICE PaulMeyerer, Munich, Germany, assignor to Siemens & HalskeAktiengesellschaft Berlin and Mumch, a corporation of GermanyApplication August 20, 1956, Serial No. 605,083 Claims priority,application Germany September 22, 1955 5 Claims. (Cl. 313-84) Thisinvention relates to wave coupling for traveling wave tubes and the likeand is particularly concerned with a device for coupling a wave guide ina hollow conductor into a tubular guide disposed between and connectedto permanent magnets.

Timing tubes and especially traveling wave tubes frequently require ahomogeneous magnetic field of relatively great length for guiding anelectron beam. Such homogeneous magnetic field is produced by the use ofa soft iron cylindrical member, constituting a tubular guide, whichinterconnects the poles of permanent magnet rods. The geometricdimensions of the soft iron cylindrical tubular guide, which ismagnetically saturated, determine the course of the magnetic fieldstrength inside thereof. The electromagnetic wave which is carried, forexample, by a hollow conductor, must be coupled into the tubular guideand for this purpose, such guide must be provided with a passage.However, such passage disturbs the rotation symmetry of the magneticfield and the corresponding disturbance may become so great duringoperation of the tube, that a considerable part of the beam currentimpinges upon the helix carrying the electromagnetic wave and disposedwithin the tubular guide, introducing the danger of destroying thehelix.

An improvement of the rotation symmetric at the coupling area may beobtained, for example, by increasing the diameter of the tubular guideat the corresponding area. Such an arrangement has the disadvantage thatit can be technically produced only for hollow conductors for a wavelength with an upper limit of about 15 cm. Folded hollow conductorsprovide in this respect no improvement. The rotation unsymmetry of themagnetic field becomes too great in the case of higher wave lengthsrequiring greater dimensions of the hollow conductor.

The object of the invention is to provide for the coupling of a wavepropagated in a hollow conductor, into a tubular guide disposed betweenand interconnecting the magnetic poles of permanent magnets, a devicewhich is also applicable for hollow conductors for wave lengthsexceeding 15 cm., substantially without disturbing the rotation symmetryof the magnetic field. The principal feature of the device according tothe invention resides in the provision of a tubular guide having tubularportions the outer ends of which are connected with the permanentmagnets and the inner ends of which are spaced apart and carry soft irondisks, thus forming an air gap at which the coupling is effected, thewidth of said air gap corresponding at least to the size of the hollowconductor, and the spaced apart portions of the tubular guide forming amagnetic impedance per unit of length of the tubular guide and the widthof the air gap which is of substantially identical magnitude.

The invention is based upon recognition of the following considerations.

When the tubular guide is by spaced apart portions thereof interruptedat the point of the coupling of the hollow conductor, the lines of forceacross the air in the resulting air gap and also in the iron will havethe same density. However, since the magnetic excitation in air is-times greater than in iron (,u=permeability of the iron), the entiremagnetic flux of the circuit will be at the air gap. What is howeverrequired is a constant field exhibiting a constancy as high as possiblealso at the point at which the tubular guide is interrupted for thepassage of the hollow conductor. This leads to the requirement for aconstant distribution of lines of force along the entire tubular guide.

In accordance with the invention, this constant distribution is obtainedby interrupting the tubular guide by the provision of an air gap formedby spaced apart portions thereof along which the magnetic impedance perunit of length of the tubular guide and air gap width is ofsubstantially equal magnitude. The cross-sectional area of the tubularguide is for this purpose advantageously increased -fold at the air gap.This increase of the crosssectional area is in accordance with theinvention in simple manner obtained by mounting on the spaced apartportions of the tubular guide soft iron magnetic disks. The mutuallyoppositely positioned surfaces of these disks compel the distribution ofthe lines of force, and a substantially homogeneous course of the fieldwill be obtained by making the disks ,u-times greater than thecross-sectional area of the tubular portions of the guide facing the airgap. The size of the soft iron disks will depend upon the permeabilityof the material of the tubular guide and the permeability of thematerial of which the disks are made. It is of advantage to use amaterial for the soft iron disks which has a permeability exceeding thatof the material of the guide tube.

The various objects and features of the invention will appear from thedescription which is rendered below with reference to the accompanyingdrawing, wherein Fig. 1 shows an example of the invention inlongitudinal cross-sectional view; and

Fig. 2 shows a transverse sectional view taken along line 22 of Fig. land looking in the direction of the arrows.

Referring now to the drawing, numeral 1 indicates permanent magnetsinterconnected by soft iron bars 2 and by spaced apart portions 3 of atubular guide adapted to receive a traveling wave tube (not shown), andformingan air gap 4. Secured to the inner end of each portion 3 of thetubular guide is a soft iron magnetic disk as respectively indicated at6 and 7. The magnetic lines of force extending within the tubular guidefrom pole to pole of the permanent magnets and across the air gap areindicated in Fig. 1 by dash lines 5. It will be seen that the density ofthe lines of force in the iron of the and 7 reduce the density of thelines of force across the air gap as compared with the density thereofin the tubular guide. By making the operatively effective area of thedisks ,u-times greater than the cross-sectional area of the iron of thetubular guide portions facing the air gap, the density of the lines offorce across the air gap will be reduced by the factor ,u, as comparedwith the density of the lines of force in the iron of the tubular guide,thereby realizing the intentions of the invention.

The disks may have the simple form as indicated in connection with thedisk 7. It is however of advantage to make the disks in the shape asindicated in connection with the disk 6, that is, with a cross-sectionwhich is greatest at the point of connection with the tubular guide anddiminishing with increasing distance laterally thereof.

The invention is not inherently limited to the shapes of the disks asshown in the drawing.

Changes may be made within the scope and spirit of the appended claims.

I claim:-

1. An electron beam focusing device for a traveling wave tube havingprovision for coupling wave energy to Said tube, said device includingspaced permanent magnets, and a paramagnetic tubular guide extendingbetween said permanent magnets, said tubular guide comprising twotubular portions, the outer end of each portion connected to one of saidpermanent magnets, the inner ends of said portions being' spaced apartand each inner end carrying a soft iron disk radially outwardlyextending therefrom, said inner ends with said disks thereon defining anair gap the width of which corresponds at least to the size of thehollow conductonthe wave coupling being effected at said air gap, saidtubular guide and air gap having a substantially constant magneticimpedance per unit of distance intermediate said permanent magnets.

2. A device according to claim 1, wherein the operatively effectiveareas of said disks are at the air gap -times greater thanthe'correspondingly positioned crosssectional area of the respectivetubular guide portions, ,1 designating the permeability of the materialof the tubular guide.

3. A device according to claim 2, wherein the respective materials ofsaid tubular guide portions and of said disks have differentpermeability.

4. A device according to claim 3, wherein the permeability of thematerial of said disks is greater than that of the material of thecorresponding tubular guide portions.

5. A device according to claim 1, wherein the crosssectional area of atleast one of said disks diminishes with increasing distance radially ofthe corresponding tubular guide portion.

References Cited in the file of this patent UNITED STATES PATENTS Veithet a1. July 22, 1958

